In many applications, a slurry comprising solids suspended in a liquid (i.e., flowable) carrier is deposited on a moving conveyor, such as the forming fabric in a papermaking process or on a filtration belt in a waste filtering process, to form a layer of solids on the conveyor. In the present application this layer of solids is termed a web, but this term is to be deemed to include a layer of particulate solids of any type or kind and which includes interstices which contain a liquid. The solids may be in the form of fibers, such as papermaking fibers or ceramic fibers; in the form of continuous or discontinuous filaments; in the form of particulate solid(s) such as filtered waste; or some combination of these. In all instances related to the present invention, the layer of solids is porous, meaning that the liquid carrier of the slurry from which the layer of solids derived will flow through at least the thickness of the layer of solids. Most commonly, the liquid carrier involved is aqueous, but nonaqueous carriers are known and reference herein to removal of liquid is deemed to include removal of other flowable carrier materials.
Drying is commonly defined as being the removal of a liquid from a material by means of heat. No heat is required in the present invention, however, a layer of solids which has been treated in accordance with the present invention may be subsequently treated with heat to dry the layer of solids. Further, the present invention contemplates the application of heat to the layer of solids in the course of removing the liquid from the layer of solids employing the present invention.
In the prior art, removal of liquid from a web (layer of solids) is commonly accomplished by applying vacuum to one surface of the web, while the web is carried on a porous conveyor, to draw liquid from the web, through the conveyor and to a collector. One variation of this technique is to simultaneously apply a pressure to that surface of the web opposite the conveyor to enhance the movement of the liquid through, and out of, the web, such as through the use of a press roll. A further technique of the prior art is to overlay the upper surface (opposite the conveyor on which the web is supported) with an absorbing web which pulls liquid from the wet web into the more dry overlaid web, that is, a type of blotting effect is accomplished. Other prior art workers have proposed to utilize capillary action to withdraw the liquid from the wet web.
In U.S. Pat. No. 4,556,450 there is disclosed a technique for removing water from a wet web which employs a capillarity effect. In this disclosure, a wet web is overlaid on a segment of a cylinder. The cylinder is provided with a porous cover or, alternatively, is provided with a porous woven fabric, either of which encircles and forms the outer wall of the cylinder. In accordance with the disclosure in this patent, the pores in the cylinder cover (be it a porous cover or a woven fabric cover) are filled with water so that there will be hydraulic communication between the water in the pores of the cylinder cover and the water held in the, interstices (pores) of the web. The "effective" diameter of the pores of the cylinder cover are said to be "finer", i.e. smaller, than that of the pores of the web so that there allegedly occurs capillary-type transfer of water from the web into the pores of the cover. In this prior art technique, however, the water which is said to transfer from the web into the pores of the cylinder cover is captured within the pores of the cylinder cover and is not allowed to pass fully through these pores. But rather, the water captured in the pores of the cover is blown by pressurized air reversely through and out of the pores once the pores have ceased to be in hydraulic engagement with the wet web. Because of this limitation of the prior technique, only very small quantities of water may withdrawn from the web via each pore. Further, in this prior technique, it is required that each of the pores in the cylinder cover contain some liquid at all times so as to prevent the flow of air into the cylinder body which results in the expenditure of excess energy in the process. This requirement also requires that the apparatus employed include extensive, hence expensive, vacuum and pressure systems which have very accurate controls, such that any vacuum developed within the cylinder body is not sufficient to draw the water from the pores in the cylinder cover, and that the pressure used to eject a portion of the water from each pore be less than that which will drive all the water from each pore.
In some applications, the economics of the liquid removal process dictate relatively harsh differential pressure conditions. In some applications, these harsh conditions may be tolerated, but in some applications, papermaking and like operations, for example, it is desired that the bulkiness of the web not be reduced materially during the water removal process. This objective can not be met by the prior art water removal techniques which employ merely a differential pressure established across the thickness of the web for the reason that these techniques condense the thickness of the web and destroy its bulk. For example, a tissue web formed of papermaking fibers desirably exhibits a maximum bulk and much effort has been expended in the papermaking industry to achieve such bulk. The present invention provides for the removal of liquid from a wet web economically under conditions ranging from harsh to those conditions where there should be no sacrificing the bulk of the web.
Economic removal of liquid from a wet web has been hindered in the prior art, first, by reason of the difficulty of accommodating the conflicting requirement that rapid withdrawal of liquid from the wet web, which heretofore has been considered to call for the use of relatively open porous conveyors for the web, with the requirement that the withdrawn liquid not be allowed to flow reversely through the pores of the conveyor and back into the web. This problem of the prior art is particularly important where the liquid withdrawal action is caused to take place in the nip between a solid "top" roll and a porous "bottom" roll as the wet web on its conveyor is passed through such nip. In this latter situation, due in part to time constraints, the liquid which is withdrawn from the web can not be carried away from the web conveyor sufficiently fast to prevent accumulation of the withdrawn liquid on the backside of the conveyor where the liquid is in position to reversely flow through the very open pores of the conveyor and reenter the web.
A further problem of the known prior art methods and apparatus for withdrawing liquid from a wet web relates to the difficulty of cleaning the equipment employed in the liquid removal process. For example, the application of a differential pressure across the thickness of a wet web during liquid removal causes fibers or particulates to move from the web and into the pores of the conveyor which is supporting the web or into the pores of any overlaid absorbing web, or the like. Most liquid removal processes must be continuous to be economical so that the web conveyor, added absorbing webs, and the like commonly are continuous loops or porous rolls as referred to hereinabove. This requires that these devices be cleaned of debris which is deposited on and within the pores of the device during the liquid removal phase, so that the device will be suitable for again supporting a fresh portion of the web and removing the liquid therefrom as the process continues. Water jets for spraying water against the moving conveyor, the porous surface of a roll, or similar device, brushes acting on the surfaces of the devices, and like techniques have been used to clean these devices. Each such prior art cleaning technique is less than effective in that eventually the devices plug and must be removed for extensive cleaning or replacement. The present invention avoids the cleaning problem by precluding the deposition or accumulation of debris on the web conveyor or related like devices during the course of removing liquid from a wet web.
It therefore is an object of the present invention to provide a method for the withdrawal of liquid from a wet web.
It is another object of the invention to provide a method for the withdrawal of liquid from a wet web wherein the withdrawn liquid is prevented from reversely flowing back into the web.
It is another object of the invention to provide a method for the withdrawal of liquid from a wet web wherein the bulk of the wet web may be maintained or reduced as desired.
It is another object of the invention to provide apparatus for the withdrawal of liquid from a wet web.
It is another object of the invention to provide apparatus for the withdrawal of liquid from a wet web and which includes means for precluding the reverse flow of the withdrawn liquid back into the web.